In a normal natural gas liquefaction process, as illustrated in FIG. 1, high pressure natural gas from which acid gases such as CO.sub.2 and H.sub.2 S are removed is cooled to approximately 20.degree. C. in a shell and tube heat exchanger 1 through which HHP propane is passed so that a majority of the water content in the natural gas may be removed and separated in a drum 2. Then, the water content is further reduced to the order of 1 wt ppm in a dryer 3, and the natural gas is cooled to 0.degree. C. in a shell and tube heat exchanger 4 through which HP propane is passed. The natural gas is further cooled in a shell and tube heat exchanger 5 through which MP propane is passed, and is cooled in a shell and tube heat exchanger 6 through which LP propane is passed before it is supplied to a scrub column 7 where heavy fractions are removed.
Then, as illustrated in FIG. 2, the natural gas is cooled to -145.degree. C. and liquefied by exchanging heat with a mixed refrigerant in a main heat exchanger 8. This stream is flashed twice in drums 9 and 10 so as to be removed of its N.sub.2 content, and is fed to a storage facility by a pump 11 as LNG at its boiling point under the atmospheric pressure.
Meanwhile, in the mixed refrigerant cycle, as illustrated in FIG. 2, after the mixed refrigerant has exchanged heat with the natural gas in the main heat exchanger 8, the mixed refrigerant is fed to a LPMR compressor 12 at 3 bar, -30.degree. C., and it is pressurized to 13 bar by the compressor 12, and cooled to the ambient temperature in an after-cooler 13. It is then pressurized to 25 bar in a HPMR compressor 14, and again cooled to the ambient temperature in an inter-cooler 15 before it is further pressurized to 40 bar by the HPMR compressor 14. The thus pressurized mixed refrigerant is cooled to the ambient temperature in an after-cooler 16, and is then further cooled to 15.degree. C. by HHP propane in a shell and tube heat exchanger 17, to 0.degree. C. by HP propane in a shell and tube heat exchanger 18, to -10.degree. C. by MP propane in a shell and tube heat exchanger 19, and to -25.degree. C. by LP propane in a shell and tube heat exchanger 20.
In this case, the mixed refrigerant starts partial condensation in the shell and tube heat exchanger 17, and is three quarters condensed in the shell and tube heat exchanger 20. It is then introduced into a separation drum 21 where the separated gas and liquid are passed through the main heat exchanger 8 for exchanging heat with the natural gas.
Now consider an example of an LNG plant with a capacity of 2.6 million tons per year. The (kettle type) shell and tube heat exchangers 1, 4, 5 and 6 that are to be cooled by propane are each required to be a large kettle type heat exchanger on the order of 1,000 to 2,000 m.sup.2, and the shell and tube heat exchangers 17, 18, 19 and 20 are each required to be a large kettle type heat exchanger on the order of 2,000 m.sup.2 .times.2. Such heat exchangers are so large in size that they are not suitable for land transportation, and the cost for the foundation and other construction work is substantial.
Further, since the natural gas or the mixed refrigerant enters these shell and tube heat exchangers 5, 6, 18, 19 and 20 in mixed phases, the liquid to gas ratio of the stream in each part of the tubes deviates so much from a theoretical value that the performance of the heat exchangers inevitably drops.